﻿//--------------------------------------------------------------------------
// 
//  Copyright (c) Microsoft Corporation.  All rights reserved. 
// 
//  File: ConcurrentPriorityQueue.cs
//
//--------------------------------------------------------------------------

using System.Collections.Generic;
using System.Diagnostics;

namespace System.Collections.Concurrent
{
    /// <summary>Provides a thread-safe priority queue data structure.</summary>
    /// <typeparam name="TKey">Specifies the type of keys used to prioritize values.</typeparam>
    /// <typeparam name="TValue">Specifies the type of elements in the queue.</typeparam>
    [DebuggerDisplay("Count={Count}")]
    public class ConcurrentPriorityQueue<TKey, TValue> :
        IProducerConsumerCollection<KeyValuePair<TKey,TValue>> 
        where TKey : IComparable<TKey>
    {
        private readonly object _syncLock = new object();
        private readonly MinBinaryHeap _minHeap = new MinBinaryHeap();

        /// <summary>Initializes a new instance of the ConcurrentPriorityQueue class.</summary>
        public ConcurrentPriorityQueue() {}

        /// <summary>Initializes a new instance of the ConcurrentPriorityQueue class that contains elements copied from the specified collection.</summary>
        /// <param name="collection">The collection whose elements are copied to the new ConcurrentPriorityQueue.</param>
        public ConcurrentPriorityQueue(IEnumerable<KeyValuePair<TKey, TValue>> collection)
        {
            if (collection == null) throw new ArgumentNullException("collection");
            foreach (var item in collection) _minHeap.Insert(item);
        }

        /// <summary>Adds the key/value pair to the priority queue.</summary>
        /// <param name="priority">The priority of the item to be added.</param>
        /// <param name="value">The item to be added.</param>
        public void Enqueue(TKey priority, TValue value)
        {
            Enqueue(new KeyValuePair<TKey, TValue>(priority, value));
        }

        /// <summary>Adds the key/value pair to the priority queue.</summary>
        /// <param name="item">The key/value pair to be added to the queue.</param>
        public void Enqueue(KeyValuePair<TKey, TValue> item)
        {
            lock (_syncLock) _minHeap.Insert(item);
        }

        /// <summary>Attempts to remove and return the next prioritized item in the queue.</summary>
        /// <param name="result">
        /// When this method returns, if the operation was successful, result contains the object removed. If
        /// no object was available to be removed, the value is unspecified.
        /// </param>
        /// <returns>
        /// true if an element was removed and returned from the queue succesfully; otherwise, false.
        /// </returns>
        public bool TryDequeue(out KeyValuePair<TKey, TValue> result)
        {
            result = default(KeyValuePair<TKey, TValue>);
            lock (_syncLock)
            {
                if (_minHeap.Count > 0)
                {
                    result = _minHeap.Remove();
                    return true;
                }
            }
            return false;
        }

        /// <summary>Attempts to return the next prioritized item in the queue.</summary>
        /// <param name="result">
        /// When this method returns, if the operation was successful, result contains the object.
        /// The queue was not modified by the operation.
        /// </param>
        /// <returns>
        /// true if an element was returned from the queue succesfully; otherwise, false.
        /// </returns>
        public bool TryPeek(out KeyValuePair<TKey, TValue> result)
        {
            result = default(KeyValuePair<TKey, TValue>);
            lock (_syncLock)
            {
                if (_minHeap.Count > 0)
                {
                    result = _minHeap.Peek();
                    return true;
                }
            }
            return false;
        }

        /// <summary>Empties the queue.</summary>
        public void Clear() { lock(_syncLock) _minHeap.Clear(); }

        /// <summary>Gets whether the queue is empty.</summary>
        public bool IsEmpty { get { return Count == 0; } }

        /// <summary>Gets the number of elements contained in the queue.</summary>
        public int Count
        {
            get { lock (_syncLock) return _minHeap.Count; }
        }

        /// <summary>Copies the elements of the collection to an array, starting at a particular array index.</summary>
        /// <param name="array">
        /// The one-dimensional array that is the destination of the elements copied from the queue.
        /// </param>
        /// <param name="index">
        /// The zero-based index in array at which copying begins.
        /// </param>
        /// <remarks>The elements will not be copied to the array in any guaranteed order.</remarks>
        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
        {
            lock (_syncLock) _minHeap.Items.CopyTo(array, index);
        }

        /// <summary>Copies the elements stored in the queue to a new array.</summary>
        /// <returns>A new array containing a snapshot of elements copied from the queue.</returns>
        public KeyValuePair<TKey, TValue>[] ToArray()
        {
            lock (_syncLock)
            {
                var clonedHeap = new MinBinaryHeap(_minHeap);
                var result = new KeyValuePair<TKey, TValue>[_minHeap.Count];
                for (int i = 0; i < result.Length; i++)
                {
                    result[i] = clonedHeap.Remove();
                }
                return result;
            }
        }

        /// <summary>Attempts to add an item in the queue.</summary>
        /// <param name="item">The key/value pair to be added.</param>
        /// <returns>
        /// true if the pair was added; otherwise, false.
        /// </returns>
        bool IProducerConsumerCollection<KeyValuePair<TKey, TValue>>.TryAdd(KeyValuePair<TKey, TValue> item)
        {
            Enqueue(item);
            return true;
        }

        /// <summary>Attempts to remove and return the next prioritized item in the queue.</summary>
        /// <param name="item">
        /// When this method returns, if the operation was successful, result contains the object removed. If
        /// no object was available to be removed, the value is unspecified.
        /// </param>
        /// <returns>
        /// true if an element was removed and returned from the queue succesfully; otherwise, false.
        /// </returns>
        bool IProducerConsumerCollection<KeyValuePair<TKey, TValue>>.TryTake(out KeyValuePair<TKey, TValue> item)
        {
            return TryDequeue(out item);
        }

        /// <summary>Returns an enumerator that iterates through the collection.</summary>
        /// <returns>An enumerator for the contents of the queue.</returns>
        /// <remarks>
        /// The enumeration represents a moment-in-time snapshot of the contents of the queue. It does not
        /// reflect any updates to the collection after GetEnumerator was called. The enumerator is safe to
        /// use concurrently with reads from and writes to the queue.
        /// </remarks>
        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
        {
            var arr = ToArray();
            return ((IEnumerable<KeyValuePair<TKey, TValue>>)arr).GetEnumerator();
        }

        /// <summary>Returns an enumerator that iterates through a collection.</summary>
        /// <returns>An IEnumerator that can be used to iterate through the collection.</returns>
        IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }

        /// <summary>Copies the elements of the collection to an array, starting at a particular array index.</summary>
        /// <param name="array">
        /// The one-dimensional array that is the destination of the elements copied from the queue.
        /// </param>
        /// <param name="index">
        /// The zero-based index in array at which copying begins.
        /// </param>
        void ICollection.CopyTo(Array array, int index)
        {
            lock (_syncLock) ((ICollection)_minHeap.Items).CopyTo(array, index);
        }

        /// <summary>
        /// Gets a value indicating whether access to the ICollection is synchronized with the SyncRoot.
        /// </summary>
        bool ICollection.IsSynchronized { get { return true; } }

        /// <summary>
        /// Gets an object that can be used to synchronize access to the collection.
        /// </summary>
        object ICollection.SyncRoot { get { return _syncLock; } }

        /// <summary>Implements a binary heap that prioritizes smaller values.</summary>
        private sealed class MinBinaryHeap
        {
            private readonly List<KeyValuePair<TKey, TValue>> _items;

            /// <summary>Initializes an empty heap.</summary>
            public MinBinaryHeap()
            {
                _items = new List<KeyValuePair<TKey, TValue>>();
            }

            /// <summary>Initializes a heap as a copy of another heap instance.</summary>
            /// <param name="heapToCopy">The heap to copy.</param>
            /// <remarks>Key/Value values are not deep cloned.</remarks>
            public MinBinaryHeap(MinBinaryHeap heapToCopy)
            {
                _items = new List<KeyValuePair<TKey, TValue>>(heapToCopy.Items);
            }

            /// <summary>Empties the heap.</summary>
            public void Clear() { _items.Clear(); }

            /// <summary>Adds an item to the heap.</summary>
            public void Insert(TKey key, TValue value)
            {
                // Create the entry based on the provided key and value
                Insert(new KeyValuePair<TKey, TValue>(key, value));
            }

            /// <summary>Adds an item to the heap.</summary>
            public void Insert(KeyValuePair<TKey,TValue> entry)
            {
                // Add the item to the list, making sure to keep track of where it was added.
                _items.Add(entry);
                int pos = _items.Count - 1;

                // If the new item is the only item, we're done.
                if (pos == 0) return;

                // Otherwise, perform log(n) operations, walking up the tree, swapping
                // where necessary based on key values
                while (pos > 0)
                {
                    // Get the next position to check
                    int nextPos = pos / 2;

                    // Extract the entry at the next position
                    var toCheck = _items[nextPos];

                    // Compare that entry to our new one.  If our entry has a smaller key, move it up.
                    // Otherwise, we're done.
                    if (entry.Key.CompareTo(toCheck.Key) < 0)
                    {
                        _items[pos] = toCheck;
                        pos = nextPos;
                    }
                    else break;
                }

                // Make sure we put this entry back in, just in case
                _items[pos] = entry;
            }

            /// <summary>Returns the entry at the top of the heap.</summary>
            public KeyValuePair<TKey, TValue> Peek()
            {
                // Returns the first item
                if (_items.Count == 0) throw new InvalidOperationException("The heap is empty.");
                return _items[0];
            }

            /// <summary>Removes the entry at the top of the heap.</summary>
            public KeyValuePair<TKey, TValue> Remove()
            {
                // Get the first item and save it for later (this is what will be returned).
                if (_items.Count == 0) throw new InvalidOperationException("The heap is empty.");
                KeyValuePair<TKey, TValue> toReturn = _items[0];

                // Remove the first item if there will only be 0 or 1 items left after doing so.  
                if (_items.Count <= 2) _items.RemoveAt(0);
                // A reheapify will be required for the removal
                else
                {
                    // Remove the first item and move the last item to the front.
                    _items[0] = _items[_items.Count - 1];
                    _items.RemoveAt(_items.Count - 1);

                    // Start reheapify
                    int current = 0, possibleSwap = 0;

                    // Keep going until the tree is a heap
                    while (true)
                    {
                        // Get the positions of the node's children
                        int leftChildPos = 2 * current + 1;
                        int rightChildPos = leftChildPos + 1;

                        // Should we swap with the left child?
                        if (leftChildPos < _items.Count)
                        {
                            // Get the two entries to compare (node and its left child)
                            var entry1 = _items[current];
                            var entry2 = _items[leftChildPos];

                            // If the child has a lower key than the parent, set that as a possible swap
                            if (entry2.Key.CompareTo(entry1.Key) < 0) possibleSwap = leftChildPos;
                        }
                        else break; // if can't swap this, we're done

                        // Should we swap with the right child?  Note that now we check with the possible swap
                        // position (which might be current and might be left child).
                        if (rightChildPos < _items.Count)
                        {
                            // Get the two entries to compare (node and its left child)
                            var entry1 = _items[possibleSwap];
                            var entry2 = _items[rightChildPos];

                            // If the child has a lower key than the parent, set that as a possible swap
                            if (entry2.Key.CompareTo(entry1.Key) < 0) possibleSwap = rightChildPos;
                        }

                        // Now swap current and possible swap if necessary
                        if (current != possibleSwap)
                        {
                            var temp = _items[current];
                            _items[current] = _items[possibleSwap];
                            _items[possibleSwap] = temp;
                        }
                        else break; // if nothing to swap, we're done

                        // Update current to the location of the swap
                        current = possibleSwap;
                    }
                }

                // Return the item from the heap
                return toReturn;
            }

            /// <summary>Gets the number of objects stored in the heap.</summary>
            public int Count { get { return _items.Count; } }

            internal List<KeyValuePair<TKey, TValue>> Items { get { return _items; } }
        }
    }
}